The term "acrylic polymers", whenever used in the present specification and in the claims, means the homopolymers or the copolymers of the alkyl esters of the acrylic or methacrylic acid, wherein the alkyl group contains from 1 to 8 carbon atoms.
Examples of said monomers are methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, sec.butyl methacrylate, tert.butyl methacrylate, etc.
The acrylic polymers can also contain up to 50% by weight of units derived from other monomers containing double bonds such as styrene, alpha-methylstyrene, acrylonitrile, (meth)acrylamide, n-alkyl maleimide or aryl maleimide, etc. or from double-unsaturation monomers such as, for example, butadiene.
The suspension polymerization is a kind of reaction which occurs in a system in which the monomer is suspended in the form of droplets in a continuous phase and it is polymerized by using a starter of the radical type and soluble in the monomer. Generally, the continuous phase is water.
The final product consists of a suspension of polymer particles (beads) having a diameter of 0.1-1 mm, which are easily separable from water by means of centrifugation.
Generally, the ratio between the continuous phase (water) and the discontinuous phase (monomer) ranges from 1:1 to 3:1.
In the practical embodiment of this type of process it is necessary to use suspension stabilizers which prevent the coalescence of the monomer droplets in the more advanced polymerization steps.
The suspension stabilizers utilized in the most usual technique are water-soluble macromolecular compounds having affinity for the monomer which, placing themselves at the interface between organic phase and aqueous phase, form a protective film which prevents the particles from caking.
On conclusion of the polymerization the suspending agent is removed from the surfaces of the polymer particles by washing with water.
The suspending agent is a key-factor as its characteristics condition the performances of the whole process as regards both the final polymer quality and the costs.